The present invention relates to an electrical terminal for a sealed accumulator.
The invention relates to any type of sealed accumulator, in particular accumulators of the nickel-cadmium, nickel-metal hydride and lithium-ion type. These accumulators can be of a cylindrical format, a rectangular format (also known as prismatic) or semi-prismatic (oblong shape)
A sealed accumulator or sealed electrochemical generator (as these two terms are equivalent, the term accumulator will be used in this description) comprises, in a manner known per se, an electrochemical bundle comprising alternating positive and negative electrodes framing separators impregnated with electrolyte; this electrochemical bundle constitutes the core of the accumulator. Typically, each electrode is made up of a metal current collector, also called a strip, supporting the electrochemically active material on at least one of its faces. Each electrode is electrically connected to a current output which ensures electrical continuity between the electrode and the external application with which the accumulator is associated. The bundle of electrodes is arranged in a container which is sealed tight by a cover; the cover can support current output terminals of both polarities. A first current output terminal, for example the positive terminal, is generally welded onto the cover. A second current output terminal, for example the negative terminal, passes through the cover; it is generally fixed to the latter by crimping and seals electrically insulate the negative current output terminal from the cover. Electrical terminals are then connected to the current output terminals for connection to an external application; an electronic card is also typically connected to the electrical terminals of the accumulator in order to control the operation of the accumulator.
Various solutions exist in the state of the art for connecting electrical terminals to the positive and negative output terminals of the accumulator and to an electronic card.
For example, the electrical terminals can be made of nickel or nickel-plated steel; they are electrically welded onto the accumulator, by laser or ultrasound, and soldered onto an electronic card. This is for example the case with the accumulators sold by the Applicant company under the reference MP176065HD in particular (nickel terminal electrically welded onto the accumulator and soldered onto the electronic card). Nickel is however a high-resistivity material; the terminals therefore have to be bulky in order to allow the passage of high currents, in particular for high-power applications (telecom, automobile). Such terminals therefore occupy a significant amount of space, are heavy and a long time is needed for soldering onto the electronic card as the terminals take time to heat up. Moreover, soldering a nickel terminal to an electronic card can cause tin to run along the terminal and cause a short circuit between the current output terminal to which the terminal is welded and the other polarity. This phenomenon will be explained more in detail with reference to FIG. 5 by comparison with the invention.
The nickel terminals can also be designed to be screwed or electrically welded onto an electronic card.
Copper terminals also exist, but copper is more difficult to laser-weld to the accumulator and welds very poorly by electric welding. Moreover, the soldering of a copper terminal to an electronic card can also cause tin to run as in the case of a nickel terminal.
Moreover, for reasons of weight and raw material costs, it is sought to produce accumulator containers from aluminium rather than stainless steel. Aluminium has better thermal dissipation and better electrical conductivity; it is also lighter and less expensive per unit of weight than stainless steel.
However, nickel terminals weld poorly to an aluminium container because of the great thickness of nickel to be used for conducting the power; the laser weld is then “brittle”. It would then be possible to use electrical terminals of aluminium, which would weld well to the aluminium container; but such terminals cannot be electrically welded onto the electronic card as aluminium is too good an electrical conductor which interferes with the welding. These aluminium electrical terminals can also not be soldered onto the electronic card as such, because tin and aluminium repel each other.
For example, the Lithium-ion accumulators sold by the Applicant company under the reference VL7P have screw terminals for connection to each other and/or for connection to an electronic card. They are made of aluminium 1050 in the case of the positive terminal (laser-welded onto the aluminium case) and nickel-plated copper in the case of the negative terminal (crimped onto the case).
Soldering the electrical terminals to the electronic card is advantageous for use on an industrial scale as the stability of the electric contact is ensured over time.
A need therefore exists for electrical accumulator terminals with a reduced space requirement, which can be easily welded onto the current output terminals and which can be soldered onto an electronic card without risk of a short circuit.